It should use the constant current power supply to drive the load in some special situations, especially, when the semiconductor illumination source is applied. Taking the high brightness white LED (light-emitting diode) that is developing very fast in illumination components field recently as an example, LED lamps have the advantages of energy saving, long life time, environmental protection and small space occupation and high reliability compared with the traditional illuminating lamps, therefore, the LED lamps have been widely used in the field of illumination, backlight and display. In order to strengthen the above mentioned advantages of white LED, the best way to drive LEDs is to adopt the constant current driver, which enables the current flowing though the LED without being influenced by the variations of voltage and ambient temperature as well as the inconsistency of the parameters of an LED.
Generally, the electric energy comes forth in a form of voltage source in the electric power system, therefore, it is necessary to use a driving circuit (voltage to current converting circuit) to convert voltage VIN into current and then provide it to the load as showed in FIG. 1. Taking driving LED as example, when the voltage of the output terminal, namely the forward voltage VLED on the LED cluster is lower than the voltage of the voltage source, the step-down driving circuit is needed. At present, there are three kinds of the step-down driving circuits;
The first one is to adopt the traditional DC-DC step-down technology as the scheme published on “Fundamentals of Power Electronics” (“Fundamentals of Power Electronics”, Republished by Kluwer Collage Publishing Housing, 2001) showed in FIG. 2. The switch tube linking in series between the input terminal and output terminal is PMOS, an inductor L is tandem connected between the output terminal of the switch tube and LED, and a current sensing resistor Rcs is connected in series between the LED and the ground and then a capacitor C is in parallel connection with the series subcircuit formed by the said LED and the said current sensing resistor Rcs, the variation of output current is fed back to the switch tube driving circuit through the voltage drop variation of the current sensing resistor Rcs, and the corresponding control signal is produced and transmitted to the gate of the said PMOS, which maintains the slim fluctuation of output current by controlling the switch-on and cut-off of the PMOS to realize the constant current output.
The disadvantages of the circuit are the switch tube driving circuit is complicate and the PMOS has big on-resistance and low efficacy. In addition, the voltage endurance capacity of the switch tube driving circuit should be bigger than the input voltage VIN, and the cost of the switch tube driving circuit will be increased vastly when the high input voltage is applied.
The second one is to adopt the NMOS switch tube to improve the efficiency as the circuit published on product description of ZXLD1350 of Zetex Semiconductor Company showed in FIG. 3. The circuit connects orderly the voltage source VIN, current sensing resistor Rcs, inductor L, LED series and the said NMOS in series and a diode D is in parallel connection with the series subcircuit formed by the current sensing resistor Rcs, inductor L and LED series, and then a switch control circuit of an error comparator with hysteresis is used to compare voltage drop of the said current sensing resistor Rcs with a reference voltage VREF, thus the control signal is produced and transmitted to the gate of the said NMOS to realize the constant current control. The circuit uses the current on the inductor, also the current on the LED at the same, to detect the current and due to the non-abrupt change property of inductor current, the current detection is quite precise; therefore, the constant current performance is quite well. The disadvantages of the circuit are the voltage endurance capacity of the current detection circuit should be bigger than the input voltage VIN because the current detection carries through the high voltage terminal and then the cost of the switch tube driving circuit will be increased vastly when the high input voltage is applied.
The third one is based on the scheme showed in FIG. 3, as the circuit published on product description of HV9910 of Supertex as in FIG. 4. In the circuit, the current sensing resistor Rcs in the tandem loop is moved into the position between the NMOS switch tube and ground lead, a low voltage control circuit is used to replace the said switch control circuit, so as to drive the high voltage switch and the goal of cost reduction is accomplished. The low voltage control circuit comprises an error comparator that receives the voltage drop signal from the said current sensing resistor, and a control circuit to produce the control signal to transmit to the NMOS gate. The disadvantages of the circuit are that only when the NMOS is on the current can be detected and in addition, the constant current performance is not very good because of the charge/discharge affect of stray capacitor CGS of the NMOS.